In electron micrographs of P. octopus, flagellar beating is oriented in CCW sequence [1]}, [2]}
consistent with observed CW body rotation (viewed from above).
Cells \(\approx 20\)  \(\mu \) m in length, with yet longer flagella whose distal portions fold and recurve by the cell,
swim at \(200\sim 300\)  \(\mu \) m\(/\) s along helical paths.
Diametrically opposite groups (usually but not exclusively pairs) of flagella beat synchronously with ordered phase shifts between groups (Fig. REF A, SI Video 7).
In-phase coupling appears more robust in distinguished pairs (SI Video 7)
reflecting prominent symmetries of the basal architecture:
e.g. a large synistosome links BBs \(1,2\)  whose flagella beat in opposite directions (Fig. REF ).
Compatible with its benthic nature, P. octopus display the `stand' gait introduced previously (Fig. REF B) .
Swimming motion resumes spontaneously when all 8 flagella reactivate (pronking), and the normal gait is reestablished within only 2 or
3 beats. A transient state in which beating occurs in 4 of 8 flagella (staggered) is observed Fig. REF D:
in which actively beating flagella are identified by summing successive gradient images (in contrast ih Fig. REF C all 8 flagella are in motion).
We visualise the flow disturbance imparted by flagellar beating using optical flow to monitor the migration of image pixels between frames,
and estimate flow directions and magnitudes [3]} (also Computer Vision toolbox, MATLAB, The MathWorks Inc.).
Components of the \(2D\)  flow field were determined by minimizing an objective function subject to certain global (smoothness)
or local (fidelity) constraints, and pixels delineating the same object are likely to have constant intensity.
Unlike conventional PIV methods, this removes the need to seed the medium with foreign particles or tracers,
and fluid flow due to motion can be obtained directly from optical data.
Flow strengths decay rapidly away from boundaries of flagella as well as the cell body which is also in motion (Figs. REF C&D, right).
More generally, gait transitions in these primitive algae can be dramatic and involve unusual flagellar coordination in response to external stimuli,
and shall be explored further in a separate manuscript.
Here, we reserve special attention to the P. cyrtoptera pronking gait, which displays striking hydrodynamic interactions.
